Unable to force use of external 32M HFXO using S132 SDK v7.0.1

Hello,

The jitter you observe is likely delays caused by higher priority Softdevice interrupts. Please see the Bluetooth Low Energy processor usage patterns  section of the Softdevice specification for more details on what kind of delays you may expect.

Do you see the same jitter if you comment out "advertising_start()" in main()?

But querying the state using sd_clock_hfclk_is_running(p_is_running); never returns true.

The function will not be able to return anything in this case because the p_is_running pointer is NULL. Please try this instead:

uint32_t is_running = 0;

sd_clock_hfclk_request(); 

while (!is_running) {
        sd_clock_hfclk_is_running(&is_running);
    }

Have you considered using PWM or a combination of TIMER, PPI, and GPIOTE to simply bypass the CPU for this task?

Minimal sample using TIMER+PPI+GPIOTE to generate clock signal out on a GPIO pin:

https://devzone.nordicsemi.com/f/nordic-q-a/55428/nrf52840-16-mhz-clock-out-on-gpio-pin/224864#224864

Best regards,

Vidar

Thanks Vidar.

My Bad... That resolved the issue of detecting the external clock startup.

Unfortunately... 

Now that I know the external clock is running , AND have switched to using the internal PWM modules...

The jitter is much improved... BUT I still see jitter in the GPIO outputs.

Thoughts?

Adrian. 

Hi Adrian,

How much jitter are you seeing? Does it happen on all periods, or only some?

Maybe you can share the code so I can try to reproduce here?

Vidar

Hi Vidar.

Thankyou for your feedback, it has been very helpful.

I am seeing very low jitter in the PWM which is less than 0.1%.

This is good enough for us.

BUT I need to use a timer to trigger the PWM at 100Hz

This timer seems to have a 2% jitter on a 2MHz clock taken when the external clock is running.

I have raised the ISR priority to 2 and it seem better (<2% jitter).

I'm not sure if there is any other way to do that.

Adrian.

Hi Adrian,

I'm glad to hear that it accuracy has improved.

anealca said:

BUT I need to use a timer to trigger the PWM at 100Hz

This timer seems to have a 2% jitter on a 2MHz clock taken when the external clock is running.

Please correct me if I'm wrong, but I assumed you wanted this to be a continuous clock signal. If you want to start the PWM every "1/100hz" seconds to play a shorter sequence, then I would recommend you connect the timer to the PWM with PPI. The problem with using the timer interrupt to trigger the PWM task is that it will take the program several CPU cycles to enter the interrupt handler.

Attached below is an example put together to test the accuracy here, and it seems to be OK. Please try to run this on your board to see if you  get the same result.

Test code based on the ble_app_hrs example

The waveforms I captured with my logic analyzer

Vidar